Gamma variation using illumination intensity

ABSTRACT

A gamma variation of image intensity is created by varying the illumination intensity during a pulse width modulated display time period. During the pulse width modulated display time period a ramp signal may be compared with the image data to determine when pixel electrodes of the pixel array are switched. The illumination intensity may be varied in concert with ramp signal to produce a quadratic variation of displayed intensity on image data value. The illumination source could be an LED illumination source and intensity of the LED illumination source could be controlled using pulse width modulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/051,899, filed Mar. 18, 2011, entitled “Adjustment of DisplayIllumination Timing,” which is a continuation of U.S. patent applicationSer. No. 12/815,108, filed Jun. 4, 2010, entitled “Microdisplay andInterface on a Single Chip,” which is a continuation of U.S. patentapplication Ser. No. 11/873,309, filed Oct. 16, 2007, entitled“Microdisplay and Interface on a Single Chip,” now issued as U.S. Pat.No. 7,755,570, which is a continuation of U.S. patent application Ser.No. 10/831,545, filed Apr. 23, 2004, entitled “Microdisplay andInterface on Single Chip,” now issued as U.S. Pat. No. 7,283,105, whichclaims priority from U.S. Provisional Patent Application No. 60/465,364,filed Apr. 24, 2003, entitled “Microdisplay and Interface on a SingleChip,” the contents of each of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

In the past several years, microdisplays have begun to displace cathoderay tubes (CRT) in various consumer product applications and to be adesirable near-eye display in certain newer product applications. Theseapplications may include video camcorders, digital still cameras, andthe emerging area of head-mounted displays. These microdisplays includeminiature display panels made from a silicon integrated circuit“backplane” that can be viewed by a user via a lens system or anyoptical magnifier. Many microdisplays produce full color images,monochrome images, or black and white images by acting as a spatiallight modulator on light provided by a separate light source. Spatiallight modulator microdisplays may employ liquid crystal materials, suchas ferroelectric or nematic liquid crystal materials, or may utilizeother technologies such as miniature mechanical mirrors or othersuitable light modulation technology. Alternatively, microdisplays mayemit their own light by employing miniature light emitting arrays madefrom emitters such as electroluminescent phosphors or organiclight-emitting diodes (OLED). In the case of liquid crystal spatiallight modulators, they may be transmissive or reflective in nature. Inthe case of reflective spatial light modulators utilizing liquidcrystals, one suitable arrangement is known as reflective LCOS (liquidcrystal on silicon). Other arrangements, with liquid-crystal modulatorsthat are frequently transmissive, include active-matrix backplanes madefrom thin-film transistors (TFT) of either polysilicon or amorphoussilicon, or made from single-crystal silicon that has been “lifted off”of a bulk-silicon wafer, as exemplified by the microdisplay products ofKopin Corporation.

The different microdisplay technologies differ significantly in theirdrive voltage requirements. For example, the electroluminescent (EL)phosphor displays require pixel drive varying over approximately an 80 Vrange to switch a pixel from fully OFF to fully ON. EL microdisplayshave achieved such drive voltages with backplanes fabricated withdoubly-diffused MOS (DMOS) high-voltage transistors as pixel drivers.The nematic LCOS displays do not usually require voltages as high,typically needing voltage swings in the range of 9-18 V, or even as lowas 5 V. In the case of LCOS using ferroelectric liquid crystals (FLCs),microdisplay products with pixels switching through only 3.3 V arecurrently in commercial production by the applicant. The 5 V and 3.3 VLCOS microdisplays have been made with backplanes fabricated instandard-logic CMOS processes having ground rules of 0.5 μm and 0.35 μm,respectively, where the standard CMOS logic provides adequate pixeldrive voltage.

The different microdisplay technologies also differ in how they producecolor. They may generate color in a field sequential fashion or viasimultaneous generation of each of the three color fields using pixelswith color triads. Field sequential color means displaying color imagesone color field at a time. For example, a red field may be displayed,followed by a green field, followed by a blue field. If these separatecolor fields are sequenced at a sufficiently high rate, the humaneye/brain will integrate them together into a perceived full colorimage.

A further issue with microdisplays is generation of gray-scale images.It is advantageous to fabricate microdisplay backplanes as conventionalsilicon integrated circuits (ICs). Producing gray scale requires eachdisplay pixel to be capable of displaying multiple brightness levels.This can be accomplished by driving an analog-responding pixel emitteror modulator with analog circuitry. Silicon fabrication processesspecialized for analog circuitry are known, but again typically costmore than baseline digital processes. Further, design of analogcircuitry is more difficult and requires greater effort than design ofsimilar digital circuitry. Analog circuitry is susceptible to a varietyof noise and offset effects which can produce unwanted image artifactsif not carefully managed. Thus, it is desirable to provide gray-scalethrough purely digital circuitry.

A number of techniques capable of producing gray scale through digitaldrive that are suitable for microdisplays are known in the art. Forexample, fast-responding emitters and modulators such as those found inplasma displays, electroluminescent displays, light-emitting diodes, theTexas Instruments Digital Micromirror Device and othermicroelectromechanical (MEMS) devices, and ferroelectric liquid crystals(FLCs) can be driven with two-level drive in such a way that variationsin the bright/dark duty cycle are used to produce apparent gray scale.In one class of such techniques, the image data is typically separatedinto “bit planes,” ranging from the most-significant bit (MSB) planedown to the least-significant bit (LSB) plane, and the image data in thebit planes is written onto the display and held for an interval ofduration proportional to the significance. Thus, in a very simpleexemplary implementation, a pixel displaying an eight-bit monochromegray scale would be written to eight times during a video frame, andmight change state as many times. In fact, such gray scale techniquesare known to produce severe visual artifacts, especially with movingpictures. One class of such artifacts is known as dynamic falsecontouring. Reduction of such artifacts requires complex variations ofthe simple example given above, with increased data processing, and morepixel state changes. Furthermore, production of a large number of grayshades, such as 256 gray shades usually required for high-quality videoimages, results in short LSB intervals during which the pixel emitter ormodulator must be able to change states. Production of 60 Hz colorimages from three sequential color fields, each of which fieldscomprises an image with the abovementioned 256 levels may requireswitching in intervals as short as 1/(3×60×255) of a second, which isabout 22 μs. For some types of modulators, such as ferroelectric liquidcrystal modulators, maintaining response times this fast can bedifficult, especially in the lower-temperature portions of the rangesmost displays are expected to operate over.

The bit-plane family of gray scale techniques can also be used with moreslowly responding display materials such as nematic liquid crystals. Inthis case the pixel has an analog response to the RMS (root-mean-square)value of an underlying two-level electrical drive. In this case, theslow, averaging nature of the liquid crystal material prevents theoccurrence of dynamic false contouring, but another class of artifactsoccurs instead. Neighboring pixels driven to adjacent gray values mayexperience very different drive waveforms. For example, in an eight-bitgray-scale scheme, a pixel driven to gray value 128 (binary 10000000)might be driven high for approximately the first half of a video frameand low for the remainder, while another pixel driven to gray value 127(binary 01111111) might be driven low for approximately the first halfof a video frame and high for the remainder. If these two pixels arephysically adjacent to each other, as would be the case if they werepart of an image with a smoothly varying brightness, a strong lateralelectrical field would be produced at the boundary between the twopixels. This lateral or fringing electrical field often produces innematic liquid crystals a defect called a disclination. Suchdisclinations have a visual contrast to the adjacent liquid crystalmaterial, often appearing much darker, and, once formed, are slow todisappear even when the electrical drive conditions that produced themare removed. Thus, brightness variations in the images produced onnematic microdisplays driven with bit-plane type digital drive become“decorated” with undesirable dark lines, which can persist momentarilyeven when the image content is changed.

Many of the above disadvantages of bit-plane type digital gray scaledrive can be overcome by alternative two-state drive schemes that reducethe number of drive transitions per video frame. For example,pulse-width modulation (PWM) drive schemes have previously been used,for example as taught in U.S. Pat. Nos. 5,977,940, 6,249,269, 6,329,974,and 6,525,709. In these examples, each pixel has its own driver, whichis typically “reset” to a chosen digital value at the beginning of thevideo field, and are then switched once (and only once) to the otherdigital value at a time proportionate to the desired gray value.However, the previous implementations referenced above, while utilizingdigital pixel drive, have all relied on underlying analog pixelcircuitry to perform a comparison between an analog image value, storedon a pixel capacitor, and a global analog ramp voltage, with each pixelhaving a analog voltage comparator in it. Analog storage of the imagevalue was chosen to reduce achievable pixel size, since a singlecapacitor can store an 8-bit image value, replacing the function ofeight digital memory registers. These analog implementations, whileavoiding the image-artifact issues described above with respect tobit-plane type digital gray scale, all suffer from the practicaldifficulties previously described for analog circuitry.

It is against this background and with a desire to improve on the priorart that the present invention has been developed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. is a block diagram of a camera in which the microdisplay of thepresent invention could be employed.

FIG. 2 is a side view of the microdisplay of the present inventionshowing a portion of the plastic packaging cut away to reveal an LCOSunit of the microdisplay of the present invention.

FIG. 3 is a cross-sectional view of the LCOS unit of FIG. 2.

FIG. 4 is top view of the silicon backplane of the LCOS unit of FIG. 2.

FIG. 5 is a block diagram of portions of the silicon backplane of FIG.4.

FIG. 6 is a perspective view of portions of the silicon backplaneshowing the size relationship between the pixel array and the layer ofboost circuits and SRAM memory cells.

FIG. 7 is a functional representation of a portion of the siliconbackplane of the present invention.

FIG. 8 is a larger-scale functional representation of a portion of thesilicon backplane of the present invention.

FIG. 9 is a functional representation of a portion of the siliconbackplane of the present invention, showing the CDP slices.

FIG. 10 is a functional view of a single CDP slice of the siliconbackplane of the present invention.

FIG. 11 is a more detailed functional view of a single CDP slice of thesilicon backplane of the present invention.

FIG. 12 is a representation of the input signals to a comparator and aresulting pixel electrical drive signal in a CDP slice of themicrodisplay of the present invention.

FIG. 13 is a logic diagram of a circuit of the present invention that isutilized in transforming between coordinate systems while decoding fromDEF to RGB color-space.

FIG. 14 is a simplified representation of a portion of the SRAM memoryarray and its connection to data-in circuits and sense amplifiercircuits.

FIGS. 15 a, 15 b, and 15 c are schematic diagrams of boost circuits foran associated SRAM memory cell that is associated with each pixel of themicrodisplay of the present invention.

FIG. 16 is a schematic diagram of a circuit used to regulate the voltagesupplied by a voltage supply of the present invention as well as togenerate a voltage signal representative of the operating temperature ofthe microdisplay of the present invention.

FIG. 17 is an illustration of a visual artifact known as tearing that isvisible in some displays of moving images.

FIG. 18 is an illustration of portions of logic for implementing PWMgray scale in digital hardware.

FIG. 19 is a timing diagram showing the interleaving of reading andwriting operations in the present invention.

FIG. 20 is a representation of a sampling technique for image data.

FIG. 21 is a representation of a sampling technique for compressed imagedata.

FIG. 22 is a representation of a sampling technique for compressed imagedata.

FIG. 23 is a representation of a first sampling technique of the presentinvention for compressed image data.

FIG. 24 is a representation of a second sampling technique of thepresent invention for compressed image data.

FIG. 25 is a representation of a third sampling technique of the presentinvention for compressed image data.

FIG. 26 is a representation of a fourth sampling technique of thepresent invention for compressed image data.

FIG. 27 is a representation of a fifth sampling technique of the presentinvention for compressed image data.

FIG. 28 is a representation of a sixth sampling technique of the presentinvention for compressed image data.

FIGS. 29 a and 29 b are a schematic and a timing diagram, respectively,for a sense amplifier for an SRAM circuit of the present invention.

FIG. 30 is a timing diagram showing the various phases of sequentialcolor operation of the present invention.

FIG. 31 is a timing diagram illustrating the temperature variation ofthe timing of the LED illumination according to the present invention.

FIG. 32 is a timing diagram illustrating the varying of displaycharacteristics for low-temperature and high-temperature operationaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the accompanying drawings, which assist inillustrating the various pertinent features of the present invention.Although the present invention will now be described primarily inconjunction with microdisplays based on spatial light modulators, itshould be expressly understood that the present invention may beapplicable to other applications where digital interfaces to imagedisplay systems, image compression, low power SRAM, and many otherfeatures of the present invention are required/desired. In this regard,the following description of a microdisplay system is presented forpurposes of illustration and description only. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with thefollowing teachings, and skill and knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedherein are further intended to explain best modes known of practicingthe invention and to enable others skilled in the art to utilize theinvention in such, or other embodiments and with various modificationsrequired by the particular application(s) or use(s) of the presentinvention.

As can be appreciated from the background of the invention, it isdesirable to implement PWM gray scale in digital architecture. Beforedescribing how this has been done by the inventors, we first have setout below why a straightforward implementation of a digital PWMtechnique is likely to suffer from a high degree of pixel complexity.The complexity of digital implementations of microdisplays depend on thetotal number of image bits needed per pixel, which in turn depends on adisplay's gamma characteristic. Gamma (γ) is the exponent of a power-lawrelationship between display brightness and input image value. Theabovementioned “bit-plane” type digital gray scale techniques produce alinear relationship between image data value and display brightness, andthus have γ=1, as do most PWM schemes. On the other hand, typical CRTdisplays have γ≧2, which, it turns out, better matches thecharacteristics of human perception. Gamma values of about two result inbrightness steps between numerically adjacent input data that have morenearly even perceptual spacing, while for γ=1, the perceived brightnesssteps are large at the low-brightness end of the scale and small at thehigh-brightness end of the gray scale. It is generally thought that theimage-quality penalty for γ=1 is about two bits per color; that is, todisplay an image equal in quality to a standard 24-bit image on a CRTrequires 30 bits on a γ=1 display. Thus, a γ=1 display operatingdirectly from a standard 8-bit/color input signal produces a colorpalette most similar in perceptible quality to a CRT with a palette of2¹⁸=262, 144 colors, instead of the desired 16.7 million-color palette.

In the case of microdisplays generating field sequential color images,current products typically include a separate interface chip upstream ofthe microdisplay to convert the incoming standard video image data intoan acceptable format for the microdisplay. For example, a standarddigital video image signal may provide red data for a first pixel(picture element), green data for that same pixel, and then blue datafor the same pixel. This will be followed by red, green, and blue data(RGB data) for the next pixel and so forth. This is continued for eachof the pixels in a particular line in the image, followed by the nextsubsequent line in the image, and so forth. The data is typicallydelivered at an almost even rate throughout the time allotted for thedisplay of a frame, except for short horizontal blanking intervals atthe end of each line and a short vertical blanking period at the end ofeach frame. For example, in the CCIR 601 and CCIR 656 standard videosignals, the horizontal blanking occupies approximately 17% of the timeallotted to each line (which time is on the order of 60 μs), while thevertical blanking occupies approximately 8% of the frame time. Theremainder of the time, data is being delivered for display. Fieldsequential color displays, on the other hand, typically require firstthe red data for each of the pixels in the image, followed by the greendata for each of the pixels in the image, followed by the blue data foreach of the pixels in the image. In the simplest sequential-colordisplay illumination schemes, the entire display is illuminated with asingle primary color at one time. In this case, all the data for a givenprimary color must be written to the pixels before the illumination cancommence, which further aggravates the data-supply problem, requiringthat the data be provided to the display at a high rate for a shortinterval of time, to avoid unduly reducing the illumination duty factor.For these reasons, field sequential color microdisplay systems requireadditional circuitry to receive the data in one format and supply it tothe microdisplay in a different format. This format conversionnecessarily requires a considerable amount of buffer memory—at least thesubstantial fraction of a memory capable of storing all the red, green,and blue data for all the pixels in the displayed image. With movingimages, additional buffer memory is required to prevent the “tearing”artifact illustrated in FIG. 17. The figure illustrates the image on asequential-color display, the display being refreshed from a singleframe buffer that is simultaneously being updated with a new incomingframe. The depicted object is moving (horizontally, in this example),which causes its position to change from frame to frame. Since thedisplay refresh rate is different (i.e. three or more times higher) thanthe update rate, the refresh and the update cannot be entirelysynchronized, and it is therefore unavoidable that portions of the imagedata corresponding to a present frame and to a previous frame appearsimultaneously on different regions of the display. Horizontal linesalong which there is a mismatch in the position of the displayed objectseparate these regions. The object's details or texture will appear tobe “torn” along these lines. This artifact is quite obvious andobjectionable to the average viewer. Avoiding it requires doublebuffering the image data, i.e., using one buffer memory to store anddisplay the previous frame, while a second buffer memory is updated withincoming image data. The role of the two buffers is reversed betweenincoming frames.

One way to provide the needed additional data reformatting or reorderingand image buffer circuitry practiced in the art is to supply it onsemiconductor chips separate from the microdisplay. A disadvantage ofthis separate interface chip approach is the increased cost due to theneed for the microdisplay system to have additional chips, for exampleone extra chip for the data format conversion and another dedicated toimage buffering memory. A further disadvantage is the increased size ofa multiple-chip display system. Finally, off-chip buffering furtherrequires high-bandwidth communication between the buffer chip and themicrodisplay, which invariably produces increased power consumption.

An alternative location for the needed circuitry and buffer memory is onthe microdisplay backplane itself, perhaps within the pixel array.However, the large amount of backplane circuitry required to effectimage buffering prevents practical implementations, since the resultingbackplane would be so large and hence expensive. If the frame buffer wassimply a memory block separate from the pixels, but still on themicrodisplay backplane, the ratio of pixel array area to total backplanearea would be undesirably reduced, since it would be impractical for thepixels to cover the memory block area. Alternatively, the circuitarchitecture of the microdisplay pixels could be designed so that theneeded buffer memory for a given pixel was part of the circuitryphysically associated with and underneath that pixel. Although thisdoesn't solve the overall backplane size problem, it does avoid theunfavorable active-area ratio problem of a separate memory block, sincethe pixels now cover the memory circuits. However, this benefit comes atthe price of introducing another substantial problem. The failure of anyof the memory registers produces visible pixel defects. Redundancytechniques used in the semiconductor memory art to improve yield by“mapping” around the address of defective registers cannot easily beused to compensate for such pixel failures, since a defective pixel atone location cannot be replaced by a functioning pixel at a differentlocation.

The backplane size problem could be addressed by specialized CMOSsilicon fabrication processes, such as embedded DRAM processes, butthese processes are more expensive to fabricate. Further, DRAM requiresconstant refreshing, which adds substantial unwanted power consumption.

The impracticality of prior-art techniques for providing the desiredfully digital sequential-color format conversion entirely within amicrodisplay backplane can best be illustrated by an example. Forpurposes of illustration, consider a microdisplay capable of displayingsequential full color, with eight bits of gray scale per color. Considerfurther that the microdisplay utilizes a double image buffer, with thebuffer circuitry located within the pixel, to eliminate visual artifactsand to allow high color field rates. Although the layout size of anarbitrary pixel circuit cannot be determined exactly without carryingout a complete design, its lower bound can be estimated by assuming thatits transistors are laid out with the same density as transistors in astandard six-transistor SRAM cell. Given that the design rules andlayout for standard SRAM cells are highly optimized, it is very unlikelythat arbitrary pixel circuits could be laid out with higher density. Ina survey of leading CMOS silicon foundries performed by the applicant,it was found that the area of optimized six-transistor SRAM cellsoffered by the foundries was generally larger than 130f², where fdesignates the CMOS process ground rule (usually the finest feasiblehalf-pitch for polysilicon lines in the specified process). For example,in a 0.35 μm CMOS process, six-transistor SRAM cells generally had areasof about 16 μm². The formula a=130f² produces an SRAM area estimateslightly larger than that estimated for future processes and futureyears in the “International Technology Roadmap for Semiconductors 2002Update”, sponsored by (among others) the United States' SemiconductorIndustry Association.

In-pixel buffering and re-ordering of image data could conveniently beaccomplished with shift registers, as is known in the sequential-colordisplay art. Standard CMOS shift register cells comprising two staticlatches (each latch further comprising four transistors in the form ofcross-coupled inverters) and two transmission gates (each transmissiongate comprising two transistors) require twelve transistors per storedbit. Thus, double-buffering 24 bits of image information requires48×12=576 transistors. If these transistors could be laid out with adensity matching that of the highly optimized standard SRAM cells, theywould occupy 1536 μm² in a 0.35 μm CMOS process. Thus, just thetransistors associated with the image buffer would limit the minimumachievable pitch of square microdisplay pixels to 39.2 μm for thiscandidate CMOS process. It is known in the sequential color display artthat a stored digital image value can be converted to a pixel-durationsignal (in effect, a PWM drive signal) by using a down counter. Eachstage of the counter can be conventionally implemented using ahalf-adder and a master/slave flip-flop, with a NAND gate to detect thezero condition, as shown in FIG. 18. The half-adder includes aneight-transistor XOR gate plus a four-transistor AND gate, the masterstage includes four transistors arranged as cross-coupled inverters plusa load transistor and an enable transistor; the slave stage is the same,minus the load transistor. The NAND gate requires two transistors perinput. Thus, the counter requires 25 transistors per bit, which, for aneight-bit gray scale translates into a total of 196 transistors, afterfour transistors in the unused AND gate at the zeroth stage of thecounter are discarded. In total, then, this double-buffered PWMimplementation of 24-bit color display requires 576+196=772 transistors.This estimate ignores miscellaneous transistors needed for pixelselection, and so on. In the aforementioned 0.35 μm CMOS process, this772-transistor pixel would require more than 2050 μm², which would limitachievable square-pixel pitch to 45 μm. This pixel size estimate can becontrasted with pixel pitches found in current commercial microdisplays,which range around 12 μm. Thus, straightforward implementation ofdigital sequential-color format conversion results in pixels with areasmore than 10 times larger than commercially competitive. For a givendisplay resolution, a large pixel size results in a large backplane diesize, which correspondingly results in few backplane die per siliconwafer and low backplane die yield, compounding to give an undesirablyhigh backplane die cost.

It may be appreciated that techniques for reducing the number of bitsneeded for an image might reduce the complexity and size of amicrodisplay backplane. For example, image compression techniques suchas JPEG compression can be used to reduce the amount of memory needed tostore an image. However, these techniques typically require complexnumerical processing logic, whose additional size offsets any savings inrequired memory.

The number of image-data bits that must be stored can also be reducedthrough the technique of constraining the number of colors the displayis capable of showing to a “palette” smaller than the full 16.7 millionshades available at the full twenty-four bits per pixel. For example, ifthe number of shades were restricted to 65,536 shades, then the numberof bits needed to be stored could be reduced from twenty-four per pixelto sixteen per pixel, with a consequent reduction in backplanecomplexity. Palettizing the image, though, produces undesirable imageartifacts of its own, particularly for continuous-tone image contentsuch as is found in photographs or videos of natural scenes, since itmakes it difficult to portray smooth color and brightness variations.This problem is greatly exacerbated for PWM pixel modulators, whichproduce a linear relationship between display brightness and input imagevalue. Further reduction of the input-value palette to 16 bits wouldresult in a palette displayed at γ=1 equivalent to 2¹⁰=1024 colors on aγ=2 display, unsuitable for almost any application.

System Elements

With this in mind, we can now discuss the present invention. One exampleof an application in which the present invention may be employed is acamera 30, as shown in FIG. 1. The camera 30 may be a video camera, adigital still camera, or another type of camera. The camera 30 mayinclude an image-capturing device 32 that is capable of creatingelectrical signals representative of an image that a user may desire torecord. The electrical signals are passed from the image-capturingdevice 32 to a controller 34 which controls the function of the camera30. The camera 30 also includes user controls 36 that the user may useto select modes of operation of the camera 30. The controller 34 has theability to store the electronic signals representative of the images ina storage device such as memory/tape unit 38. In the case of a videocamera, this may typically be a videotape, while in the case of adigital still camera, this may typically be some type of electronic,non-volatile memory. The camera 30 also includes a battery 40 thatsupplies power to the components of the camera 30 via a powerdistribution unit 42. The stored electronic representation of the imagescan be converted to visual images by a microdisplay 44 that may beviewed by the user via a lens system 46 or reflective magnifier. Whilethis is one example of an application in which the microdisplay of thepresent invention may be utilized, it is only exemplary in nature and isnot intended to limit in any fashion the scope of the invention.

The microdisplay 44 is shown in FIG. 2 to illustrate its majorcomponents. The microdisplay 44 includes a plastic package housing 52 towhich an illuminator housing 54 is attached. The illuminator housing 54houses a tri-color LED 56 and a reflector 58 that collects light emittedby the LED 56. The light then passes through a pre-polarizer anddiffuser 60 to minimize stray light of unwanted polarization and tocreate even illumination. The diffuse, polarized light is directedtoward a polarizing beam splitter (PBS) 62, which reflects light of onelinear polarization while rejecting light of an orthogonal linearpolarization. The reflected light is directed down toward a liquidcrystal on silicon (LCOS) display panel 64 that resides in the packagehousing 52. As will be described in further detail below, the displaypanel includes an array of pixels that can be electronically controlledinto one of two different light-modulating states. In onelight-modulating state, the incoming polarized light is reflected backtoward the PBS 62 with the same polarization. In anotherlight-modulating state, the light is reflected back toward the PBS 62with its linear polarization rotated by 90°. As can be appreciated, thePBS 62 will reflect the reflected light that has not had itspolarization rotated, while the light that has been rotated inpolarization will pass through the PBS 62 for viewing by the user viathe lens system 46. A connector 66 depends downward from the packagehousing 52 for electrical connection to the camera 30 such as via a flexcable.

The above discussion of the operation of the display panel 64 is notintended to limit the present invention, as other types of spatial lightmodulators could also be utilized in the present invention, such asspatial light modulators depending on miniature mechanical mirrors, forexample. Also, display panels that emit their own light could be used.In addition, while the discussion involves linearly polarized light oftwo different orthogonal directions, it is also possible to utilize thepresent invention in a system in which unpolarized light or differenttypes of polarization are used. Further details on the operation ofliquid crystal spatial light modulators can be found in U.S. Pat. Nos.5,748,164, 5,808,800, 5,977,940, 6,100,945, 6,507,330, 6,525,709, and6,633,301, the contents of each of which are incorporated herein byreference.

Display Panel Detail

The display panel 64 is shown in greater detail in FIGS. 3 and 4. Asshown in FIG. 3, the display panel 64 includes a silicon backplane 70 towhich a sheet of glass 72 has been affixed via glue seal 74. Sandwichedbetween the silicon backplane 70 and the sheet of glass 72 is a layer ofliquid crystal material 76. When viewed from a different side, it maybecome clear that the glass 72 and the backplane 70 are offset slightlyin one direction to allow there to be a slight overhang of glass on oneside and a slight overhang of silicon on the opposite side. The liquidcrystal material 76 may include any of several types of liquid crystalsincluding, but not limited to, ferroelectric, nematic, or other types ofliquid crystals. In this embodiment, ferroelectric liquid crystals areutilized. Alternatively, other types of display devices such as digitalmicromirror and other microelectromechanical devices, plasma displays,electroluminescent displays, light-emitting diodes, and the like couldbe employed as part of the display panel. As can be appreciated, thesealternatives may either be spatial light modulators that modulate lightfrom a light source or they may be light emissive devices that do notrequire a separate light source.

The silicon backplane 70 includes an area on a top surface thereof wherean array 80 of reflective pixel electrodes is located. As can beappreciated, the image is formed in this area of the display panel 64,which is known as the “active area” of the display panel. The siliconbackplane 70 is shown in FIG. 3 to be formed of solid silicon materialsolely for ease of illustration of the major components of the displaypanel 64. In actuality, a plurality of circuits, conductors, and soforth exist within the silicon backplane 70, as will be discussed infurther detail below.

FIG. 5 is intended to represent certain significant portions of thesilicon backplane 70 in a functional manner and components thatinterface with the backplane 70, rather than in a positional manner. Thesilicon backplane 70 has an active pixel area 82 that includes aplurality of rows and columns of pixels. Two pixels are shown in theactive pixel area 82, a first pixel 84 and a second pixel 86. Locatedwithin the silicon backplane under the array 80 of reflective pixelelectrodes is circuitry (that will be discussed in further detailbelow), a major component of which is a plurality of memory cellsunderneath the active pixel area 82 and optionally extending beyond theboundary of the active pixel area 82 vertically as viewed in FIG. 5,where additional regions 88 and 90 of memory cells are shown. In thepresent embodiment, these memory cells are implemented as conventionalsix-transistor SRAM, although other types of memory registers, includingdynamic registers, could be used as well. Two particular areas of memorycells are illustrated in FIG. 5, a first area 92 of SRAM and a secondarea 94 of SRAM. As will be seen, the first area of SRAM 92 isfunctionally associated with the first pixel 84 and the second area ofSRAM 94 is functionally associated with the second pixel 86, althoughthe areas of SRAM 92 and 94 are not located next to the first and secondpixels 84 and 86. A second major component is a plurality of boostcircuits which are capable of storing data as an SRAM cell as well asbeing able to drive a particular voltage onto a pixel electrode ascommanded by the data stored in the boost cell.

In this embodiment, an upper-half 96 of the active pixel area 82 isassociated with one set of circuitry shown in FIG. 5 above the activepixel area 82, while a lower-half 98 of the active pixel area 82 isassociated with circuitry shown in FIG. 5 below the active pixel area82. In this case, the upper half 96 and lower half 98 are divided alonga dividing line 100 shown in FIG. 5. As will be discussed in furtherdetail below, the additional circuitry above and below the active pixelarea 82 that is associated with the upper and lower halves 96 and 98 area pair of banks of sense amplifiers 102 and 104 respectively, a pair ofbanks of pixel line-buffers and column drivers 106 and 108 respectively,and a pair of banks of column data processors (CDPs) 110 and 112,respectively. The sense amplifiers in the banks 102 and 104 read thecontents of the SRAM memory cells for use by the column data processorsof the banks 110 and 112. The pixel line-buffers of the banks 106 and108 temporarily store data on the way to the SRAM, include a circuit todrive the columns of the SRAM, and provide a mechanism to selectivelydrive only particular columns of the SRAM. The column data processors ofthe banks 110 and 112 receive data read back by the sense amplifiersfrom the SRAM, decompress and compare the data to a ramp signal 114, todetermine when and how pixel electrodes of pixel array 82 should bedriven so as to generate a displayed image.

A control unit 116 in the silicon backplane 70 receives image dataprovided to the microdisplay 44 such as image data that may come fromcontroller 34 of the camera 30 in any one of various formats. Thecontrol unit 116 is operative to accept image data in at least threedifferent standard video formats, including RGB serial, CCIR-601, andCCIR-656. In each of these standard formats, the image data associatedwith all three primary colors is transmitted for a given pixel beforeany image data is transmitted for the next pixel. The timing for each ofthese video formats can be NTSC or PAL and the vertical frequency can beeither 50 Hz or 60 Hz. The resolution of the RGB serial data may be432×240 while the resolution for the CCIR video formats may be either720×242 or 720×288. The invention is not limited to any particularformat, timing, vertical frequency, resolution, or geometry. Theinvention can further provide an analog to digital converter in theinput data path, to allow the display to accept standard analog videosignals, and provide digital data to the remainder of the display. Thecontrol unit is operative to perform gamma correction, dither, andscaling on the received images as may be necessary and appropriate. Forexample, if the column data processors and SRAM array were actingtogether to produce a PWM gray-scale with γ=1, as will be described inmore detail below, and the received image data were provided from astandard source designed to drive a γ=2 display, the control unit couldtransform the incoming digital values to new values such that whendisplayed a viewer would perceive a correct gamma characteristic. In thepresent embodiment, this is accomplished by transforming the incoming8-bit/color data to 10-bit/color data with the desired gamma correction.In order to display this data within an 8-bit/color limitation of oneembodiment of the present invention, the 10-bit/color data is convertedto 8-bit/color data using a Floyd-Steinberg error-diffusion algorithmcarried out within control block 116 to minimize the visibility of anyerrors resulting from lack of precision with which the 10-bit values canbe represented. Additionally, the control block 116 can horizontally andvertically scale the incoming image data (which might have, for example,720×242 or 720×288 formats) using bilinear interpolation to a 432×240format matching the format of the pixel array.

The control unit 116 receives a clock signal from a display clock 118.The clock signal from the display clock also drives a ramp counter 120that supplies the afore-mentioned ramp signal 114. The control unit 116controls row control logic 122, which selects which row of the pixelsand which SRAM cells will be accessed. The control unit 116 alsocommunicates with a variety of peripheral circuit elements, some ofwhich may be located separately from the backplane 70. These elementsinclude a temperature sensor 124, a window driver 125, a pixel-voltagegenerator, one or more LED drivers, one or more digital-to-analogconverters (DACs), one or more analog-to-digital converters (ADCs), anon-volatile memory such as an EEPROM 126, and a set of LEDs 127.

FIG. 6 shows the size relationship between the array of pixel electrodes80 in the layer 130 of SRAM memory cells and the pixel-driving boostcircuits lying therebeneath. As can be appreciated, in one dimension,the pixel array 80 and the layer 130 underneath are the same width,while in another dimension, the layer 130 is significantly taller thanthe pixel array 80. As illustrated in FIG. 5, this is because of theadditional SRAM 88 and 90 that is utilized in this particularembodiment.

FIG. 7 illustrates the positional relationship between the pixel array80 and the layer 130. For purposes of illustration, portions of thepixel array 80 have been removed to expose portions of the layer 130therebeneath. In this embodiment, each of the pixel electrodes 132 isone of a group of eight adjacent pixel electrodes in a single column, asshown in FIG. 7. The layer 130 underneath the pixel array 80 includes aplurality of rows of boost circuits 134 and a plurality of SRAM memorycells 136. As can be seen, the boost circuits 134 are grouped togetherin pairs of adjacent rows, separated by approximately thirty rows ofSRAM memory cells 136. Further, the boost circuits 134 are located inthis embodiment in a particular position relative to each of the pixelelectrodes 132, while SRAM occupies the remaining area between boostcircuits. The location of data in SRAM cells relative to boost circuitsand pixels in which that data will be eventually displayed isessentially arbitrary. These relationships, or lack thereof, can bebetter appreciated in FIG. 8. On the left side of FIG. 8, eight pixelelectrodes, of which four 146, 148, 150, and 152 are in a first verticalcolumn and another four are in a second vertical column, can be seenwith a plurality of boost circuits 138, 140, 142, and 144 shown inphantom underneath the pixel electrode 146. Additional boost circuitsand SRAM memory cells are shown in phantom underneath the secondvertical column of pixel electrodes. At the right side of FIG. 8, thesilicon backplane 70 is seen with the pixel electrodes removed todirectly expose a plurality of pixel boost circuits 134 and SRAM memorycells 136.

In this embodiment, a group of four particular boost circuits 138, 140,142, and 144 in a particular row are associated with four particularpixel electrodes 146, 148, 150, and 152 in a particular column. Sincethe boost circuits occupy a space that is approximately ¼ of the widthof a pixel electrode, the four boost circuits 138, 140, 142, and 144 lieunderneath a single one 146 of the pixel electrodes. In this embodiment,boost circuit 138 is associated with and drives pixel electrode 146,boost circuit 140 is associated with and drives pixel electrode 148,boost circuit 142 is associated with and drives pixel electrode 150, andboost circuit 144 is associated with and drives pixel electrode 152. Ascan be appreciated, the remaining space under pixel electrode 146 andall of the space under pixel electrodes 148, 150, and 152 is occupied bya plurality of SRAM memory cells 136. With this particular sizing of thepixel electrodes 132, however, and the particular semiconductorfabrication process used, the remaining space under particular pixelelectrodes 146, 148, 150, and 152 is not sufficient for the bufferedstorage required by this design for those four pixel electrodes. Forthis reason, additional space vertically above and below the pixel array80 is used for additional SRAM 88 and 90 in the layer 130 as shown inFIGS. 5 and 6.

Referring back to FIG. 7, it can be appreciated that the bottom row ofeach adjacent two rows of boost circuits 134 is associated with anddrives four pixel electrodes—the pixel electrode in that same columnthat is located directly on top of the bottom row of boost circuits andthe three adjacent pixel electrodes lying therebeneath in the samecolumn. Similarly, the upper row of each pair of boost circuits 134 isassociated with and drives four pixel electrodes—the pixel electrode inthat same column that is located directly on top of the upper row ofboost circuits and the three adjacent pixel electrodes vertically abovein the same column.

As previously described, in this embodiment more SRAM storage isrequired than will fit beneath the active pixel array, in the spacebetween the boost circuits. For this reason, the SRAM extends beyond theactive pixel array 80 as indicated in FIG. 6. It can be appreciatedthen, that there exists no specific relationship between a pixelelectrode of the active pixel array and the SRAM locations where datafor that pixel is stored, other than that the SRAM used must reside inthe same vertical slice as the pixel electrode, which will be explainedfurther on.

It can be appreciated that the lack of a specific relationship betweenthe location of the particular SRAM memory cells 136 and a particularpixel electrode 132 is advantageous because it allows the microdisplay44 to utilize SRAM memory cells that are in the additional SRAM areas 88and 90 as a memory buffer for image data to be displayed on a pixelelectrode anywhere in the pixel array 80. In addition, should it bedetermined that particular SRAM memory cells 136 or rows of SRAM memorycells are defective, spare SRAM memory cells or rows of SRAM memorycells located elsewhere in the silicon backplane 70 can be utilized as amemory buffer for that particular pixel electrode 132. The addresslocation of the bad cells or rows can be stored at the time of anexternal die test in the EEPROM non-volatile memory associated with themicrodisplay, or determined by a built-in self-test function thatlaunches when the microdisplay is powered up and stored in volatileregisters in the microdisplay. Logic within the control block 116 canretrieve and interpret the addresses of the bad cells or rows, andautomatically substitute the addresses of suitable spare cells or rows.

Column Data Processor and its Functions

Portions of the silicon backplane 70 are illustrated in FIG. 9. As hasbeen previously discussed, the silicon backplane 70 is divided into anupper half and a lower half by a dividing line 100. The upper halfincludes a horizontal bank 102 of sense amplifiers, a horizontal bank106 of pixel line buffers and column drivers, and a horizontal bank 110of column data processors (CDPs). Further, it can be seen that thisportion of the silicon backplane is divided into vertical slices, ofwhich three of the slices, 160, 162, and 164 are shown. Each verticalslice has its own CDP and associated pixel line buffers and columndrivers from the bank 106 as well as associated sense amplifiers fromthe bank 102. The CDP of a particular slice performs the data processingfor each of the pixel electrodes in that given slice. On the other sideof the dividing line 100, similar CDP slices 166, 168, and 170 areshown. Each of these slices 166, 168, and 170 include a single CDP andassociated pixel line buffers and column drivers from the bank 108 andassociated sense amplifiers from the bank 104.

SRAM Reading and Writing

FIG. 10 shows a functional representation of the CDP slice 160. Encodedimage data 172 from the control unit 116 is provided to a column driver174 in the slice 160. The column driver 174 passes data past a senseamplifier 176 (which is deselected via a signal 178 for this writeoperation) to a plurality of SRAM memory cells 180. The particular SRAMmemory cell in which the data is stored is determined by the particularcolumn driver 174 and a row enable signal 182 from the row control unit122. Later, the control unit 116 will command the row control unit 122to drive the row enable signal 182 for the row of SRAM cells 180 wherethe pixel electrode's data is stored and enable sense amplifier 176 byactivating the sense amp enable signal 178. The sense amplifiers 176will determine the contents of the selected SRAM cells 180 and pass thedata to the CDP 186 of slice 160. The CDP 186 decompresses the data andcompares selected portions of the decompressed value to the ramp signal114, the result of which is then stored temporarily and then laterwritten to the boost circuit 188 that is associated with pixel 184, bydisabling the sense amp 176, enabling the column driver 174, and causingthe row control unit 122 to drive row enable signal 190.

Digital Pulse-width-modulation Gray Scale

FIG. 11 shows more detail about the operation of reading data from SRAM,performing a comparison and using the result of the comparison todetermine when to change the state of the pixel. In this case, the SRAMmemory cells are represented as an entire row of memory cells that mayinclude 48 different memory cells in this CDP slice 160. The particularrow 180 is selected by the row enable signal 182. The data fromparticular memory cells in the SRAM memory 180 is read by the senseamplifiers 176 and supplied to a decode block 200, which will bediscussed in further detail below. The decode block 200 receives adecode signal 202 from the control unit 116. The decode signal 202indicates which portion of the result decoded from stored encoded imagedata should be provided to a first input of a digital comparator 204.The comparator 204 compares this decoded portion of the encoded imagevalue to the digital ramp signal 114 (which is provided to its secondinput) and provides (via logic described below in conjunction with FIG.12) a pixel control signal 206 to a multiplexer 208 that can becontrolled to enable writing of either the pixel control signal 206 orthe encoded image signal to storage locations below. In this case, thepixel control signal 206 is provided to the selected boost circuit 188that is in a group of four boost circuits in a column underneath fourpixels, 184, 210, 212, and 214 (for ease of illustration, the boostcircuits are not shown underneath one of the pixels in this case). Boostcircuit 188 is associated with and is in electrical communication withthe pixel electrode 184, whose state the pixel control signal 206 isintended to control. In this embodiment, each boost circuit functions asa one-bit storage register for the desired state of the pixel. It can beseen that eight boost circuits are shown in the row including boostcircuit 188. These are grouped into 2 groups of four boost circuits,with the left-most group of four boost circuits storing the intendeddisplay value for the four pixels shown therebelow 184, 210, 212, and214, and the right-most group of four boost circuits storing theintended display value for the four pixels 216, 218, 220, and 222 showntherebelow. For ease of later discussion, a two-by-two array of pixels184, 210, 216, and 218 is indicated in FIG. 11 as pixel group 224 whileanother group of four pixels 212, 214, 220, and 222 is shown in FIG. 11as pixel group 226.

In this particular embodiment, the pixel array 80 includes 240 pixelsvertically and 432 pixels horizontally. The dividing line 100 separatesthis 240×432 array into two arrays of 120 pixels vertically and 432pixels horizontally. Each of these two arrays is sliced vertically, ashas been previously described, into 36 CDP slices. Each of these CDPslices includes a sub-array of 120 pixels vertically and 12 pixelshorizontally. Underneath these pixels are rows of boost circuits, with48 boost circuits in each particular row of a CDP slice, or 4 boostcircuits for each column of pixels. In between the double rows of boostcircuits, there are approximately 30 rows of SRAM memory cells, withthere being 48 memory cells in each row of each CDP slice. This isintended as but one embodiment of the present invention, and none of thesizes or numbers discussed herein are intended to limit the invention.

FIG. 12 shows the signals input to the comparator 204 in the CDP slice160, the output 232 of the comparator 204 that helps to create the pixelcontrol signal 206, and also the resulting pixel electrical drive signal228 that represents the state of the pixel electrode. The ramp signal114 is one of the inputs to the comparator, while the other input is adecoded pixel value 230. The pixel electrical drive signal 228 is shownjust below, on the same time scale. Both comparator input signals aredigital, although they are shown pictorially in this figure in a waywhere vertical direction in the figure indicates value. The ramp signal114, output by ramp counter 120, might be the output of an eight-bitcounter, progressing from binary value 00000000 (digital 0) to binaryvalue 11111111 (digital 255). Schemes other than simple counting, suchas gray codes, can also be used. Further, the count can proceed indescending order as well as ascending order. As can be appreciated fromthe figure, when the ramp signal 114 begins, the pixel electrical drivesignal 228 transitions from, for example, a low to a high state, assignaled by the pixel control signal 206. Once the ramp signal 114reaches the same digital value as the decoded pixel value 230, thecomparator output 232 goes high. This output enables column driver 174to write to a pixel selected by row enable signal 182, thereby providingpixel control signal 206. In this case, the value of pixel controlsignal 206 has been selected so that the pixel electrical drive signal228 transitions from a high state to a low state in response to thedetection of equality by comparator 204. Of course, the length of timethe pixel electrical drive signal 228 is in a high state relative to ina low state is a function of the magnitude of the decoded pixel value230. In other words, for small magnitudes of the decoded pixel value230, the high state of the pixel electrical drive signal 228 will berelatively short, while for relatively large magnitudes of the decodedpixel value 230, the pixel electrical drive signal 228 will be in a highstate for a substantial majority of the time. In this way, themicrodisplay 44 implements pulse-width modulation pixel drive. Ofcourse, the sense of the PWM can be reversed (i.e. relatively longdurations of pixel electrical drive signal 228 for small magnitudes ofdecoded pixel value 230, and so on, by simple changes to the logiccircuitry). The functionality of the column data processors isaccomplished via combinatorial logic such as adders, shifters, and thelike.

It may be preferable for the pixel control signal 206 to be written tothe boost circuits for each pixel in a different order for the top halfof the display versus the bottom half of the display. For example if thesignal were written to each half in the same manner, updating the imagefrom top row to bottom row in each half, the portion of the image nearthe dividing lines 100 between the two halves would have portions of anold image next to a portion of the new image a substantial majority ofthe time. This may cause visible image artifacts. In order to avoidthis, it is possible to update the top half from top row to bottom row,while updating the bottom half from bottom row to top row. This may alsobe called updating from outside to inside. Alternatively, the updatingcould be done in exactly the opposite fashion, from inside to outside,or from bottom row to top row in the top half and from top row to bottomrow in the bottom half. Another alternative would be to go from outsideto inside in one frame of data, from inside to outside in the nextsucceeding frame, and repeating in this alternating fashion. Anyvariations on these alternatives or any other that eliminates the visualartifacts would be acceptable.

An important feature of the microdisplay disclosed here is its abilityto accept and simultaneously display standard video signals. This isaccomplished by the aforementioned banks of pixel line buffers 106 and108, in conjunction with the action of column data processors 110 and112, as is now described with respect to FIG. 19. This figure shows thetiming relationship between elements of the input video signal, namelythe encoded input video data and a VALID signal, and elements of thegray-scale action of the CDP and pixel line buffers, namely the displayclock 118, a READ signal supplied by the control block 116 to the pixelline buffers 106 and 108 and the sense amplifiers 102 and 104, and asequence of row addresses supplied by control block 116 to row control122. The READ signal has one state (e.g., high) when it is desired forthe sense amplifiers to be reading the data from a row of SRAM registersselected by row control 122, and the opposite state (e.g., low) when itis desired to write encoded image data to selected rows of the SRAMregisters. As previously described, the SRAM array provides doublebuffering of the encoded image data so that while data corresponding toan incoming image may be written to one buffer, data corresponding to aprevious image can be read without being corrupted from a second buffer.For the purposes of FIG. 19, row addresses in one image buffer aredenoted B0Ri, where i denotes the row number, while row addresses in theother buffer are denoted B1Ri. Row addresses corresponding to pixelboost circuit registers are denoted PRi. At the time depicted at theleft of FIG. 19, VALID is high indicating that valid image data is beingsupplied as this data is supplied it is temporarily written to theregisters in pixel line buffers 106 and 108. Meanwhile, READ is high andthe CDPs are processing stored encoded image data for gray-scale displayon the pixel array. In the example depicted in FIG. 19, encoded imagedata is being read from buffer bank BO, starting with row B0R1 at theleft of the figure. After twelve clocks, data has been read from rowsB0R1 through B0R12, after which time the CDP has accumulated enoughcomparison results to write back pixel control signal 206 to registersin the boost cells. To effect this, READ goes low at this point,completing a cycle of reading frame buffer registers and updatingpixels. As depicted in the figure, VALID might go low during this cycle,marking the end of a line of incoming video data. Control block 116 thusrecognizes that the pixel line-buffers are full. Even though the nextcycle of reading of image data from the frame buffer has already begun(reading from lines B0R13 and B0R14 in the example portrayed in thisfigure), the buffer-full signal causes the cycle to be interrupted, READgoes low, and the data stored in the pixel line-buffers is written tothe other frame-buffer bank (to rows B1R1 through B1R6 in this example).After this writing completes, READ goes high, and the normal cycle ofreading followed eventually by writing to the boost registers continues.In this way, by interleaving the reading of data from one frame bufferwith occasional writing of data to the other frame buffer, the displaycan simultaneously accept standard video data while displaying anartifact-free image. Given that a new line of incoming video data startsevery 60 ms or so, and that the horizontal blanking portion of thisperiod occupies about 11 ms, and that the time required to empty thepixel line-buffers is equal to six display clock periods, or about 100ns (for the 60 MHz display clock typically used by the applicant), therequirement to write incoming data occurs relatively infrequently, andcan be caused to occur anywhere in a relatively wide interval, andcauses only minimal perturbation of the gray scale scheme.

Image Compression/decompression

One of the features of the present invention is that the incoming imagedata provided to the control unit 116 can be compressed for storagepurposes in the SRAM image buffer distributed throughout the siliconbackplane 70 and decompressed for eventual display by the pixels. Any ofseveral different types of compression algorithms are available toaccomplish this. One approach is to first convert RGB data from red,green, and blue values for each pixel to the conventional YUV system ora variant thereof. The YUV system includes a luminance component (Y) andtwo color-difference signals (U and V). In one common version, denotedYC_(B)C_(R), the color difference signals store largely red (C_(R)) andblue (C_(B)) information where luminance (which also contains most ofthe green) has been subtracted out. The following matrix transformationgenerates standard YC_(B)C_(R) signals from RGB signals:

$\begin{matrix}{\begin{bmatrix}Y \\C_{B} \\C_{R}\end{bmatrix} = {\begin{bmatrix}16 \\128 \\128\end{bmatrix} + {{\frac{1}{256}\begin{bmatrix}65.738 & 129.057 & 25.064 \\{- 37.945} & {- 74.494} & 112.439 \\112.439 & {- 94.154} & 18.285\end{bmatrix}} \cdot \begin{bmatrix}R \\G \\B\end{bmatrix}}}} & (1)\end{matrix}$

In this representation R, G, and B take on values from 0 to 255(unsigned 8-bit numbers). Y ranges from 16 to 235, and C_(B) and C_(R)range from 16 to 240. In some implementations, the YC_(B)C_(R) valuesare restricted to a subset of the 8-bit range (0-255) to permit theinsertion of special codes for synchronization and to allow processingheadroom in video electronics.

One can invert this transformation to restore RGB values (which arerequired to actually view an image, on a CRT monitor, for example):

$\begin{matrix}{\begin{bmatrix}R \\G \\B\end{bmatrix} = {{\frac{1}{256}\begin{bmatrix}298.082 & 0 & 408.583 \\298.082 & {- 100.291} & {- 208.120} \\298.082 & 516.411 & 0\end{bmatrix}} \cdot \begin{bmatrix}{Y - 16} \\{C_{B} - 128} \\{C_{R} - 128}\end{bmatrix}}} & (2)\end{matrix}$

One motivation for using the YUV system rather than other similarsystems is that the human visual system has different responses todifferent wavelengths of light. The ability to discriminate fine spatialdetail, for example, is higher for images where the detail is in theluminance than it is for images where the luminance is more constant andthe detail is in color variations. Spatial resolving power is also lowerfor blue than it is for red or green. The compression algorithm of thepresent invention takes advantage of this difference in spatialresolving power based on color. The algorithm converts the RGB data to avariant of the YUV system.

Existing standard sampling techniques are denoted by terms such as 4:4:4(illustrated in FIG. 20), 4:2:2 (FIG. 21), and 4:1:1 (FIGS. 22 and 23).In YUV-type systems having a first component containing luminanceinformation and the next two components containing color difference orsome other type of chroma information, the three numbers in the term4:2:2 express the rate at which each of those components is sampled. InFIGS. 20 and 21, each square represents a separate pixel with a separateluminance sample for each pixel. In FIG. 20, these squares alsorepresent separate U and V values for each pixel. In FIG. 21, eachrectangle with thick-lined borders represents two adjacent pixels thattogether have a single U value and a single V value. Thus, in 4:2:2 theY component is sampled twice as often as the color differencecomponents, and in 4:1:1, the Y component is sampled four times as often(as demonstrated by the thick-lined rectangles that each include fourpixels as shown in FIGS. 22 and 23). The phrase 4:2:2 is often called“broadcast video” and is considered a fairly high quality imagecompression format. Modern-day consumer digital video camcorders use4:1:1 almost exclusively. This reduced sampling normally occurs within agiven horizontal line of image data. Thus, for a scan line including 720pixels, a 4:1:1 sampling technique would imply 720 luminance (Y)samples, 180 C_(R) samples, and 180 C_(B) samples, as shown in FIG. 22.This is the NTSC version of 4:1:1. PAL systems typically also includevertical sub-sampling. Instead of four horizontal pixels sharing asingle C_(R) sample, for example, a 2×2 region of pixels shares a singleC_(R) sample, as shown in FIG. 23. This requires the addition of a linebuffer and digital video systems to store previous scan lines, but itproduces slightly more pleasing images. Thus, the PAL version of 4:1:1is sometimes denoted as 4:2:0 to emphasize this difference in samplinggeometry.

The present invention receives 24-bit RGB data (8 bits each for red,green, and blue) for each pixel and converts it to a format to bediscussed further below that can be stored as an average of 12 bits perpixel. As will be seen, the pixels are grouped into 2×2 pixel groups,such as pixel group 224 and 226, so for each pixel group, 48 bits ofdata are stored for each image. Because of double buffering, two 48-bitrows of data are required for each pixel group.

Furthermore, in order to simplify the data processing at the encodingstage performed by the control unit 116 and at the decoding stageperformed by decode blocks in the CDPs, such as decode block 200, a newvariant on the YUV system, called DEF, has been created. The coordinatetransformations areD _(i)=(½)R _(i)+(½)G _(i)E=(−¼)R _(ave)+(−¼)G _(ave)+(½)B _(ave)F=(½)R _(ave)+(−½)G _(ave)  (3)for the forward transformation andR _(i) =D _(i) +FG _(i) =D _(i) −FB _(i) =D _(i)+2E  (4)for the reverse transformation. Here the i subscript denotes values forsingle pixels while E and F are based on R_(ave), G_(ave), and B_(ave)values averaged over several pixels.

D, E, and F are three letters arbitrarily selected to represent this newcolor-space that is a variant of the YUV system. The letters have noparticular meaning other than they seek to avoid use of letters commonto other color-space schemes such as RGB, YUV, C_(R), and C_(B). Notethat the coordinate transformations can be performed with integerarithmetic rather than the floating-point arithmetic that would berequired to convert between the RGB and YUV formats as shown inEquations 1 and 2 above. Since the DEF color space is intended only asan interim color-space for the purpose of storage of images internal tothe microdisplay, the meaning of what D, E, and F represent is somewhatarbitrary, unlike the YUV system.

As an alternative to the sampling described above, that requires a framebuffer of effectively 12 bits per pixel, it would also be possible tosample at a 12:2:1 format to effectively require a frame buffer of 10bits per pixel.

As will be understood, referring back to FIGS. 5 and 11, the image dataprovided to the control unit 116 may be 24 bit RGB data while theencoded image data 172 provided from the control unit 116 to the linebuffer and column driver banks 106 and 108 is in DEF format (with 48bits coding the image content associated with four pixels, for anaverage of 12 bits per pixel). The data in this DEF format is thenstored in the SRAM memory cells 180 in the vertical slices and laterread by the sense amplifiers 176 and provided to the decode blocks 200of the vertical slices where the DEF data is converted back to RGB data,prior to the aforementioned comparison operations.

There are many alternatives to the type of sampling that could beutilized in the present invention. This may also include many variantsto the 4:1:1-equivalent coding, such as one in which each of the 2×2pixel groups is aligned with the 2×2 pixel group therebeneath (FIG. 23),or the 2×2 pixel group in the next adjacent pair of rows to the first2×2 pixel group could be offset horizontally by one pixel (FIG. 24) sothat the 2×2 pixel groups are not vertically aligned. Another variant(shown in FIG. 25) would be to define different 2×2 pixel groups for oneof the chroma components (e.g., the E or the F component) (signified bythe thick-lined borders) and define a different 2×2 pixel group for theother of the two chroma components (signified by the absence or presenceof cross-hatching). In other words, the pixel group for the E componentwould share only two pixels with a pixel group for the F component.Otherwise, the E component pixel groups would be aligned with each othervertically and the F component pixel groups could be aligned with eachother vertically. As a further variation on this variation, there couldbe an offset by one pixel horizontally in every other pair of adjacentrows (FIG. 26) so that the E component pixel groups were not alignedvertically, nor are the F component pixel groups aligned vertically.Another variation would be to define a four-pixel group that is not a2×2 array (FIG. 27). For example, a pixel group could consist of threepixels on one row and one pixel in an adjacent row, to achieve an Lshape. The next adjacent pixel group could have three pixels on theadjacent row and one pixel on the original row to also achieve an Lshape and make the two L shapes mate together into a combination of thepixel groups which is two pixels high and four pixels wide. Thisarrangement could be done for both of the chroma components, or only onewith the other chroma components having the original 2×2 configuration,for example. As can be seen, there are nearly endless variations ofchroma combinations. There are several methods to these variations, oneis to stagger the starting positions of the color difference samples inthe vertical direction. This is intended to address the appearance ofvertical striping that can occur in the image if there is too muchvertical correlation of samples in the compression technique. Anothermethod is to displace the two chroma samples relative to each other. Yetanother method is to vary the type of sampling geometries. Also, sincethe human visual system is relatively less sensitive to blue light, theE component that has the blue light as a sub-component could be sampledat an even lower rate that the F component. One approach would be a12:2:1 sampling technique that would require an average 10 bits perpixel. In this case (shown in FIG. 28), every pixel would have its own Dvalue, while a 3×2 pixel group would share an F value and a 6×2 pixelgroup would share an E value.

Thus, it can be understood that since each row of SRAM within aparticular vertical slice contains 48 bits of data, which represents theencoded luminance and chroma information (in the defined DEF format) fora 2×2 pixel array or pixel group, and that it is desired to write anentire row or rows of pixels at the same time (actually it is desired towrite to one row of boost circuits which corresponds to four rows ofpixels), there needs to be 12 different reads, each of a different48-bit row to get all the information needed to decode, compare, andwrite the desired state to four of the rows of pixels. As thecomparisons are performed, their results are gradually stored in a48-bit register. After this register is full (of 48 comparison results),the accumulated values are used to enable (in the case that the resultof a particular comparison was equality) or not enable (in the case thatthe result of a different particular comparison was inequality) thewriting of a change to the boost circuit register in a single writeoperation.

A power-saving feature of the microdisplay 44 is that the data in theregister acts as a write enable to the boost circuits and thus onlycauses a change to the bit lines in one of the 0 or 1 conditions.Because of this, the number of times that the bit lines need to becharged/discharged is reduced.

Since E and F are signed numbers between −127 and 128 and D is anunsigned number between 0 and 255, it is possible to have valid DEFvalues that transform (via Equation 4) to invalid RGB values (e.g., R,G, or B has a value less than 0 or greater than 255). Performingclipping in the traditional manner by comparing the transformed valuesto 0 and 255 and taking action if they exceed the acceptable range ispossible, but it is likely to consume too much silicon real estate. FIG.13 shows a simplified clipping circuit that is included as part of eachdecode block 200 to prevent the generation of any values outside of therange of 0 to 255 by the DEF to RGB conversion. As can be seen fromEquation 4, the DEF to RGB conversion requires as inputs D_(i), 2E, andF. A decode signal 202 is provided in, for example, on two lines calledGREEN and BLUE. When the CDP is providing green decoded image data tothe comparator, GREEN is active and BLUE is inactive. When the CDP isproviding blue decoded image data, BLUE is active and GREEN is inactive.When the CDP is providing red decoded image data, both GREEN and BLUEare inactive. GREEN and BLUE both being simultaneously active isavoided. A first multiplexer selects between 2E and F depending on thestate of BLUE, with the multiplexer output being provided as a firstinput to a summer The signal D is provided as the other input to thesummer, which also accepts the signal GREEN at its carry input. Theoutput of the summer is provided to a second multiplexer. A carry outputof the summer is provided as an input to an exclusive OR gate. The otherinput to the exclusive OR gate is provided from a third multiplexer thatreceives the sign bits from E and F (the MSB of each). The thirdmultiplexer is controlled by the BLUE decode signal so that when blue isto be decoded, the sign bit of E is used and otherwise, the sign bit ofF is used. Thus, the selected sign bit and the carry bit from the summerare inputs to the exclusive OR gate and the output will be a logical 1if the inputs are different and a logical 0 if the inputs are the same.This output and its inverse are provided as the two inputs to a fourthmultiplexer. The fourth multiplexer is controlled by the GREEN decodesignal so that when green is to be decoded, the inverted output of theexclusive OR is used and otherwise the inverted output of the exclusiveOR is used. If the output of the fourth multiplexer is a logical 0, itmeans no clipping is necessary and the 8-bit output of the summer isused. If the output is a logical 1, however, it means clipping isnecessary and instead of the output of the summer, the carry bit isselected (expanded to 8 bits of the same value of the carry bit, ofcourse). Thus, either a 255 (binary 11111111) or a 0 (binary 00000000)is provided.

One aspect of the present invention is the logical separation of datastorage in the distributed frame buffer from the storage register tocontrol the display of the pixel. These two storage locations arelogically separated while a common physical access mechanism (the CDP,the sense amps, column drivers, and row control unit) functionallyinterrelates the two storage areas.

Gray-scale Modes

The microdisplay of the present invention may provide 120 full colorimages per second, which means 120 red images, 120 green images, and 120blue images due to its field sequential color nature. This essentiallymeans that it displays 360 images per second, which means a new image orat least a new color field every 1/360^(th) of a second, or 2.78milliseconds. During each of these 2.78 millisecond intervals, encodeddata is read from the SRAM memory cells, decoded, and compared 255 timesto the ramp signal 114. Each of these 11360^(th) slices of a second isthus divided into 255 time slots of the ramp signal 114. This meansthere are at least 360×255 time slots per second. Thus each time slotis, at most, 10.9 microseconds long. During each of these time slots,new data may be written to the storage register of each pixel to changeits state in this digital pulse width modulation approach.

While a 3× (3 times the input field frequency) mode displaying 8 bits ofeach of three colors with 512 data comparisons per color (256 to bedisplayed and 256 for DC-balance) has been described, the microdisplay44 also allows for several other display modes. One is a 6× modedisplaying 7 bits of each of three colors. This mode has 512 datacomparisons per color per field. Another is a 6×8-bit Split MSBs 7-4mode. By displaying only 6 bits in each of the two display ramps, thismode delivers 8-bit gray scale resolution in a display field with thelowest power algorithm available. The first algorithm cycle has 32 datacomparisons, the second algorithm cycle has 64 data comparisons, for atotal of 192 data comparisons per color per field. Another is a 6×8-bitAdd LSB mode. This mode runs in 7-bit mode during the first algorithmcycle and 8 bits during the second algorithm cycle. Color values withthe LSB on switch one cycle later than with the LSB off This produces awaveform where the LSB is added to the second 7-bit ramp. This mode has512 data comparisons per color per field.

SRAM

FIG. 14 shows a portion of the array of SRAM memory cells in the layer130 of the silicon backplane 30. For ease of illustration, only nine ofthe SRAM memory cells are shown in three rows, each having three cellsthat are arranged in columns with the corresponding cells in theadjacent rows. The SRAM memory cells in FIG. 14 are labeled SRAM_(XY)with X being the row number and Y being the column number for the SRAMmemory cell. Each column of SRAM memory cells has a pair of BIT linesassociated therewith, BIT_(Y) and BIT _(Y), where Y is the column numberfor the BIT lines. Each row of SRAM memory cells has a word line,Word_(X), where X is the row number for the word line. Associated witheach column of SRAM memory cells is a single “data-in” circuitdesignated as DI_(Y), where Y is the column number and a sense amplifiercircuit designated as SA_(Y), where Y is the column number. The row ofdata-in circuits DI_(Y) each receives a data-in enable signal (DIE) thatserves to enable the entire row of data-in circuits. Each senseamplifier circuit (SA_(Y)) receives an amplifier enable signal (SAE)that enables the entire row of sense amplifiers. The data-in DA_(Y) andsense amplifier SA_(Y) are also connected to the BIT_(Y) and BIT _(Y)lines as this is how data is written to and read from the SRAM memorycells. Each data-in circuit DI_(Y) receives a separate data signal D_(Y)indicating the data to be written to the selected SRAM memory cell. Eachsense amplifier circuit SA_(Y) provides a sense amplifier output signalSAO_(Y) indicative of the value read from the selected SRAM memory cell.

For example, although data could be written to or read from anyindividual SRAM memory cell without regard to other SRAM memory cells inthat same particular row, it is most typical to write data to an entirerow of SRAM memory cells at the same time and to read data from anentire row of SRAM memory cells at the same time. If it were desired towrite data to the second row shown in FIG. 14, then data D_(Y) would beprovided to each of the DI_(Y) circuits and the data-in enable DIEsignal would be set to a logical 1. The Word₂ line would also be set toa logical 1 so that the data D_(Y) can be put on the BIT_(Y) and BIT_(Y) lines by the data-in circuits DI_(Y). The second row of SRAM memorycells being enable by the Word₂ line being at a logical 1, will accessthe BIT_(Y) and BIT _(Y) lines and store the value therewithin, as willbe described further below. The Word₂ and DIE signals can then bereturned to a logical 0. When it is desired to read data from the secondrow of SRAM memory cells, the Word₂ line is set to a logical 1 and thesecond row of SRAM memory cells SRAM_(2Y) provides information on theBIT_(Y) and BIT _(Y) lines to be read by the sense amplifier circuitsSA_(T). Once the sense amplifier enable signal SAE is set to a logical1, the sense amplifier circuits SA_(Y) become activated, read theinformation on the BIT_(Y) and BIT _(Y) lines and provide an outputsignal at the SAO_(Y) lines.

Low-power Features

It is important in many microdisplay applications to minimize powerconsumed by the microdisplay. The presently disclosed microdisplayincorporates a number a features to minimize the contribution of SRAMoperation to overall microdisplay power consumption, which contributionwould otherwise be impractically large. It is known in the memory art toseek to minimize the power drawn by SRAM by using current-modeoperation, as is demonstrated by Khellah, “A Low-Power High-PerformanceCurrent-Mode Multiport SRAM” IEEE Transactions On VLSI Systems, Vol. 9,No. 5, pp. 590-98 (October 2001) and Blalock and Jaeger, “A High-SpeedClamped Bit-Line Current-Mode Sense Amplifier” IEEE Journal ofSolid-State Circuits, Vol. 26, No. 4 (April 1991), the contents of whichare incorporated herein by reference. In current-mode operation, bothBIT_(Y) and BIT _(Y) lines are held at a nearly unchanging voltagelevel, and differential current injected (during writing) or detected(during reading) into the lines is used to operate the memory. Bykeeping the voltage swing V of the bit lines small, CV² powerdissipation caused by charging and discharging the capacitance C of thebit lines is kept small. In the architecture of the presentmicrodisplay, many read operations occur for each write operation, sothe power consumption during reading is substantially more important foroverall display power consumption. The applicants attempted to applycurrent-mode teaching during the design of the present microdisplay, butfound that current-mode sense amplifiers known in the art were not wellsuited to use in applicant's microdisplay. Prior-art current-mode senseamplifiers were difficult to lay out on the tight pitch required by themicrodisplay SRAM column spacing. Further, the bias current required foradequate sense-amplifier sensitivity resulted in large sense-amplifierpower dissipation for the SRAM array of applicant's microdisplay. Thus,the use of current-mode operation defeated its low-power purpose in thisapplication.

Novel low-power design and operation of the SRAM array of the presentmicrodisplay are illustrated in FIGS. 29 a and 29 b. FIG. 29 a shows thecircuit schematic for the sense amplifier 176. Sense amplifier 176 actsas a precision voltage comparator. SRAM bit lines BIT_(Y) and BIT _(Y)are connected the sense amplifier inputs at the gates of transistors N43and N44. The amplifier operates as follows with reference also to FIG.29 b. Prior to a read, the sense amplifier enable signal SAE is heldlow, blocking any current flow in the amplifier, while pulling internalnodes V1 and V2 high to V_(DD). The bit lines are also pulled high toV_(DD) by the action of P45 and P46 under the control of signal PRE.Prior to the SRAM read, PRE goes high, letting the bit lines goopen-circuit. Next the Word line is pulsed high to connect the SRAMregister in the selected row to the bit lines. One side of the SRAM cellis already high, but the other side will begin pulling one of the bitlines low ( BIT _(Y) in the example shown here). The Word line pulse iskept short to limit the voltage swing of the bit line. In typicaloperation, the width of the Word line pulse might be 4 ns, during whichthe bit line comes down on the order of 200 mV. Next the sense amplifieris enabled by signal SAE going high. This releases the internal nodes V1and V2, and also causes current to flow through N45 and N46. Given thata small differential voltage (on the order of 200 mV in this example)appears between the gates of N43 and N44, one of the V1 and V2 internalnodes will fall faster than the other. Feedback generated by the crosscoupling of V1 to the gates of N42 and P42 and of V2 to the gates of N41and P41 will cause the sense amplifier to rapidly latch into a statedetermined by the small voltage difference between the bit lines. Theoutput of the sense amplifier the reveals the state originally stored inthe selected SRAM register.

The key features of sense amplifier 176 are that it operates in voltagemode with very little power consumption, and that it is amenable to avery compact layout. Power consumption during reading of the SRAM arrayis minimized by the short pulsed action of the Word lines which servesto minimize the developed voltage swing on the bit lines, therebykeeping CV² power dissipation low. By limiting the bit-line swing to 200mV CV² power dissipation is reduced by a factor of 150 compared to aconventional SRAM operating in a mode where the bit lines swung all theway to the V_(DD)=2.5 V rails typical for a 0.25 μm CMOS process.

Other techniques are used in the SRAM array of the present microdisplayin order to further reduce power consumption. Cutting the array in halfalong the dividing line 100 helps to save power and clock distribution.Limiting the number of write cycles saves on power.

Pixel Boost Circuits

More detail about the boost circuits 188 is provided in FIGS. 15 a, 15b, and 15 c. The boost circuits can be constructed from standard-logiclow-voltage transistors in cascode arrangements, as shown in FIGS. 15 aand 15 b, or by using higher-voltage I/O transistors, and shown in FIG.15 c.

Cascode Boost Circuits

The boost-circuit embodiment shown in FIG. 15 a includes a storageregister portion 260 and a boost portion 262. Each of the transistors inthe boost circuit is an enhancement-mode device. It includes a pair ofN-channel access devices N11 and N14, which are controlled by a wordline. When turned on by the word line being at a logical 1, these accessdevices N11 and N14 allow the remainder of the storage register 260 tobe connected to the BIT and BIT lines, respectively. The remainder ofthe storage register 260 includes a pair of inverters, one inverterincluding P11 and N12, and the second inverter including P12 and N13.When the word line is moved to logical 1, the voltage of the BIT line isimposed on a node 264, located between N12 and P11. Similarly, N14 isturned on and the voltage on the BIT line is imposed on a node 266between N13 and P12. Since each of these nodes 264 and 266 are connectedto the gate terminals of the opposite inverter, this condition ismaintained even after access devices N11 and N14 are turned off. Thesource terminals of P11 and P12 are connected to V_(DD). Each of N11,N12, N13, and N14 have their P-wells, the silicon substrate, connectedto ground, while P11 and P12 have their N-wells connected to V_(DD).

The gate terminals of N12 and P11 are also connected to the gateterminal of N15 in the boost portion 262. The gate terminals of N13 andP12 are also connected to the gate terminal of N16 in the boost portion262. Thus, N15 will be turned off and N16 will be turned on. The sourceterminals of N15 and N16 are connected to ground. The drain terminals ofN15 and N16 are connected to the source terminals of N17 and N18,respectively. The gate terminals of N17 and N18 are connected to a fixedbias signal VNBIAS at a voltage of 2.5 volts. The drain terminals of N17and N18 are connected to the drain terminals of P13 and P14,respectively. The gate terminals of P13 and P14 are connected tovariable voltage bias signal VPBIAS. A node 268 between the drainterminal of P13 and the drain terminal of N17 is connected to the pixelelectrode for that particular boost circuit. The source terminals of P13and P14 are respectively connected to the drain terminals of P15 andP16, respectively. The source terminals of P15 and P16 are connectedtogether and to a separate voltage supply, V_(PIX). The gate terminal ofP16 is connected to the drain terminal of P15, while the gate terminalof P15 is connected to the drain terminal of P16. Each of N15, N16, N17,and N18 have their P-wells, the silicon substrate, connected to ground,while P13, P14, P15, and P16 have their N-wells connected to V_(PIX).

In this example, gate N15 is turned off and gate N16 is turned on, andit is assumed that V_(DD) is at a value of 2.5 volts and V_(PIX) is at avalue of 4 volts. VPBIAS is variable and controllable to beapproximately 2.5 volts less than V_(PIX), with a minimum ofapproximately 0.5 volts. VNBIAS is fixed to have a voltage ofapproximately 2.5 volts above ground. The VNBIAS causes N17 and N18 tocontinuously conduct while VPBIAS causes P13 and P14 to alsocontinuously conduct. Since N16 is on, the voltage on the drain-sourceconnection of N16 and N18 and the drain-drain connection of N18 and P14is approximately zero volts. Since the gate of P14 is at a voltage ofV_(PIX) minus 2.5V, the device stops conducting when its source voltageis less than a threshold voltage of approximately 0.45V above the gatevoltage. Thus, the source voltage of P14 for this instance will be(4V−2.5V)+0.45V or 1.95 above its drain voltage of approximately zerovolts. Since the source of P14 is connected to the gate of P15, P15 willbe conducting since its source-gate voltage of 2.05V is well above therequired threshold voltage of 0.45V. Since P15 is conducting, its drainand the gate of P16 will be at approximately 4V, which will turn P16off. Since P13 is conducting because its gate is at VPBIAS, the drain ofP13 will be at the V_(PIX) voltage of 4 volts. In this manner, when P15is on and voltage of approximately V_(PIX) is imposed on the drainterminal of P13, the variable VPBIAS gate voltage will assure that thevoltage between the gate and source terminals of P13 is at 2.5 volts,while the source-drain voltage is approximately 0 volts and under nocircumstances is greater than 2.5 volts. This prevents high source-gatevoltages from overstressing and damaging P13 by hot carriers or oxidebreakdown. When P13 is on in this manner, then the voltage at the node268 connected to the pixel electrode is approximately equal to V_(PIX).At the same time, N17 with its gate biased at 2.5V stops conducting asits source approaches one threshold voltage below its gate voltage2.5V−0.45=2.05V. In this manner, high voltage damage to N17 is preventedbecause the source-gate voltage of 2.05V and the source-drain voltage of1.95V are well tolerated by the 2.5V device. Higher pixel voltages maybe controlled in a like manner by interposing isolated well N-channeland P-channel devices with corresponding carefully controlled biasvoltages to limit the maximum voltages across all source-gate andsource-drain device terminals. It can be appreciated that V_(PIX) can bevaried from a voltage at a minimum of between 1.1 and 1.2 volts and amaximum of 5 volts in this embodiment, to compensate for various effectssuch as temperature and other environmental conditions. As V_(PIX) isvaried for these reasons of compensation, VPBIAS is also varied so thatnone of the gates in the boost circuit 188 are overstressed.

An alternative cascode embodiment is shown in FIG. 15 b. As in thecircuit shown in FIG. 15 a, it includes a storage register portion 260similar to SRAM memory cell 234, and a boost portion 300. In theembodiment of FIG. 15 b, boost portion 300 comprises only fourtransistors, P21, P22, N21 and N22. The gate of N22 is connected to thegate of transistor N12 (node 266) in storage register portion 260, whichnode is at 0 V or at V_(DD) depending on whether a 0 or a 1 is stored inthe register. The gate of N21 and P22 are both connected to a biasvoltage VPBIAS which is now set at V_(PIX)/2. The gate of P21 isconnected to a separate bias voltage CUR chosen to cause P21 to act as acurrent source, sourcing a small current, for example 8 nA, towards P22.The pixel electrode is connected to the node 302 between P22 and N21.When the gate of N22 is low, N22 is turned off, and no current flowsthrough N21 or N22, VPBIAS keeps P22 turned on, and the small currentquickly charges node 302 and the pixel electrode to V_(PIX). When thegate of N22 is high, N22 is turned on, allowing current to flow toground. VPBIAS at the gate of N21 keeps it turned on, too, allowing node302 and the pixel electrode to be discharged to ground. The smallcurrent flows continuously in this state.

High-voltage-transistor Boost Circuits

As an alternative to the cascode boost circuit described above withrespect to FIGS. 15 a and 15 b, boost circuits can be implemented withtransistors, as are available in many low-voltage CMOS processes, whichare designed to operate at the higher voltage levels frequently requiredfor I/O, the transistors usually utilizing a thicker gate oxide than thecore logic transistors. Such a boost circuit is shown in FIG. 15 c. Itagain includes a storage register portion 260 and a boost portion 304.In the embodiment of FIG. 15 c, boost portion 304 is made up of fourtransistors, N31, N32, P31, and P32, each designed to stand off theentire voltage V_(PIX). Transistors useful for this purpose includetransistors provided in many low-voltage CMOS processes to perform I/Ofunctions which require voltages higher than the core logic V_(DD)value. Such transistors are typically fabricated using a gate oxidethicker than that provided for the core-logic transistors. The boostportion of the circuit is again driven by node 266 internal to theregister portion 260, which node 266 has voltages 0 or V_(DD) dependingon the value of the bit stored in the register. When node 266 is low,N31 is turned on by V_(DD) applied to its gate, and the node between N31and P31 is pulled low, turning P32 on, and pulling the pixel electrodehigh to V_(PIX). When node 266 is high, N31 is turned off, but N32 isturned on, pulling the pixel electrode low to ground, while turning P32off.

Temperature Sensors

The microdisplay 44 of the present invention also includes a temperaturecompensation scheme that can be used to compensate for variations inperformance of the microdisplay 44 and the effect on the images producedthereby as a result of operating temperature. For example, the responseof the liquid crystal material used in a microdisplay may differdepending on the operating temperature of the liquid crystal material.In this case, it may be desirable to use a different drive voltage forthe liquid crystal material to compensate for the different switchingspeed of the liquid crystal material based on temperature. By selectinga different drive voltage, it may be possible to make the liquid crystalswitching speed independent of temperature variation. As has beendescribed above, it is possible to select different drive voltages forthe pixel electrodes. A circuit 280 for sensing temperature variationsin the microdisplay 44 is shown in FIG. 16. Specifically, the circuit280 may be located within or not within the silicon backplane 70. Thecircuit 280 may be a variation on a conventional bandgap referencecircuit. Bandgap reference circuits are intended to provide a voltagethat to a first order is temperature and supply independent. In thiscase, the circuit 280 includes a group 282 of eight diodes in parallelbeing driven by a constant current source 284. The voltage developed bythe current from the constant current source 284 across the group 282 ofdiodes is provided as an input to a positive terminal of an amplifier290. A voltage is also developed by the bandgap voltage across a voltagedivider including resistors 286 and 288. The voltage across resistor 286is provided as an input to the negative terminal of the amplifier 290. Afeedback resistor 292 determines the gain of the amplifier 290. Thevoltage across the group 282 of diodes will vary from approximately 0.7volts to 0.4 volts as the temperature varies from −20 to 100 degrees C.The output from the amplifier 290 will vary from 1.6 volts to 0.0 voltsover the same temperature range. The amplifier 290 and conventionaldownstream circuitry not illustrated here are used to quantify thisvariation in voltage and control the power supply to provide a desiredvoltage supply (V_(PIX)) therefrom. The voltage is proportional to theoperating temperature of the silicon backplane, and can be used fortemperature compensation. This voltage is supplied to control unit 116that digitizes the temperature sensor voltage and compares the averagedtemperature value with stored set points. When the temperature reaches astored set point, the V_(PIX) voltage is gradually adjusted over manyframes to a voltage value stored is association with the temperature setpoint. The timing of the signals applied to the pixel electrodes may bevaried as well. It is also possible to compensate for otherenvironmental conditions by sensing them and varying the voltage ortiming of drive signals or illumination.

Display Operation

The various features of the microdisplay described above yield superiordisplayed image quality over wide operating temperature ranges withreduced power consumption, as described below.

Sequential Color Modes and DC Balance

To provide flexibility in the sequential color display, and to providefor DC balance of the liquid crystal drive signal, as is known in theart to be desirable, the microdisplay of the present invention dividesthe frame time associated with each frame of video input data into anumber of phases, for example twelve phases as shown in FIG. 30. Duringeach phase, the variables listed in the following table can beindependently controlled:

VAR. # VARIABLE VALUES 0 Illum 0 = LED off; 1 = LED on 1, 2 PhaseColor00 = Red, 01 = Green, 10 = Blue 3 PixelOn 0 = pixel switches to 0, 1 =pixel switches to 1 4 FillDir 0 = Fill outside to in, 1 = Fill inside toout 5 BlankWr 0 = Disable pixel write during blanking; 1 = enable pixelwrite during blanking 6 BlankWrDir 0 = Write pixel to 0 during blanking;1 = Write pixel to 1 during blanking (if BlankWr is enabled)

In the example of FIG. 30, the variables are programmed to take on thevalues show in the table below, for example by storing the tabulatedsequence in appropriate registers in the display's EEPROM 126.

VARIABLE PHASE 0 1 2 3 4 5 6 0 1 0 0 0 0 0 x 1 1 0 1 0 0 1 1 2 1 1 0 0 01 1 3 0 0 0 1 0 0 x 4 0 0 1 1 0 1 0 5 0 1 0 1 0 1 0 6 1 0 0 0 0 0 x 7 10 1 0 0 1 1 8 1 1 0 0 0 1 1 9 0 0 0 1 0 0 x 10 0 0 1 1 0 1 0 11 0 1 0 10 1 0

The figure shows the ramp signal 114, the interval during which pixelboost registers are being updated, the periods during which the pixelarray is illuminated by red, green, or blue light emitted by the LEDs,and the voltage of an exemplary pixel electrode, which pixel has beencommanded to display a 50% neutral gray value. For 60 Hz input video,each phase has a duration of 1.389 ms.

During phase 0, the CDP comparators act on the red portion of decodedimage data. For a period at the beginning of the phase, called theblanking period, all the pixels in the array are driven ON. The blankingperiod might typically have a duration of 400 μs. At the conclusion ofthe blanking period the ramp and comparison of decoded image databegins. At a time shortly after the beginning of the ramp, the red LEDis turned on. Halfway through the ramp, for this exemplary pixeldisplaying 50% brightness, the comparator detects equality of the pixelimage value and the ramp value, and the CDP commands the pixel to shutOFF. At the conclusion of the ramp, the LED is turned off, and all thepixels are again driven ON again in preparation for the beginning of thenext phase. Phase 1 proceeds as did Phase 0, except this time it is thegreen LED that is activated while the green portion of the decoded imagedata is applied to the inputs of the comparators. Phase 2 follows Phase1, with the blue LED and blue data. At the conclusion of Phase 1, noblanking signal is needed, since Phase 1 will be followed in thisexample by a DC-balancing phase (Phase 3, described next) which acts inan “inverse” way. For this reason, the final state of all the pixels andthe end of Phase 2 is already what's needed to begin Phase 3 without anyfurther explicit action. During Phase 3, the display again applies thered portion of decoded image data to the comparator inputs, but duringthis DC-balance phase the LEDs are kept off, and the pixel starts withits electrode low, putting the pixel in an OFF state, and it switches ONhalfway through (i.e. the sense of action of the comparators isreversed). Additional DC-balance phases for the green and blue datafollow during Phases 4 and 5. At Phase 6, the red, green blue displaycycle starts again. In this way, each color is displayed twice duringthe frame, for a duration per color of 1.389 ms. Further, regardless ofthe image data value for a given pixel, its drive electrode spend halfthe frame time high, and half the frame time low, providing aDC-balanced drive signal to eliminate image sticking as is taught inU.S. Pat. No. 6,525,709, the contents of which are incorporated hereinby reference. Denoting by R the period during which red data isdisplayed, and by r (the lower case version of the particular color) thecorresponding DC-balance period, and so on, the display of the presentinvention can be programmed to display data in the order RGBrgbRGBrgb asillustrated in FIG. 30, or in the order RrGgBbRrGgBb, or, for furtherexample, as gbrRBGgbrRBG, or in many other permutation.

Gamma Variation

As described previously, simple PWM schemes with constant illuminationand ramp clock frequencies produce a display characteristic of γ=1,while γ=2 produces perceptually superior utilization of a limited graybit depth. The display of the present invention can provide the desiredgamma characteristic in a number of ways. In a first way, the LEDintensity is ramped in concert with the ramp signal 114. This produces aquadratic variation of displayed intensity on image data value. The LEDintensity is preferable controlled by a PWM scheme, to avoid variationof spectrum with instantaneous current value.

In a second way, the LED intensity is held constant during the ramp, butthe frequency of the clock driving the ramp counter is “chirped” so thatthe interval between ramp values is relatively short for the portion ofthe ramp corresponding to dark pixel values, and relatively long for theportion of the ramp corresponding to bright pixel values. In either theLED-ramping or clock-chirping methods, a variety of gammacharacteristics can be obtained by appropriate choice of how the LEDbrightness or clock frequency is varied with ramp count.

White-point Adjustment

The ability to vary the LED intensity has another important advantage.RGB LED triads as provided have substantial variation in the relativebrightnesses of the different colors, resulting in variation of theperceived white color. This can be corrected at the time of manufactureby measuring the relative intensities of the different LEDs underreference drive conditions, and providing the results of thesemeasurements as efficiency coefficients that can be stored in theparticular display's EEPROM. Then during operation, under the action ofcontrol block 116, the relative drive strengths provided to thedifferent LEDs can be adjusted to exactly compensate for theirvariation, resulting in a consistent desired white point.

Temperature Compensation

It is desired to have microdisplays that display high-quality imagesover a wide range of operating temperatures, even though thecharacteristics of, for example, ferroelectric liquid crystal (FLC)modulators that might be employed change dramatically over the desiredtemperature range. For example, the switching speed of an FLC typicallyslows as the FLC temperature is reduced. This slow switching can causedegradation of display contrast ratio. The microdisplay of the presentinvention provides schemes for compensating these and similar effects.

LED Timing

A first compensation method involves temperature variation of the timingof the LED illumination, as is illustrated in FIG. 31. This figure showsa portion of one display phase for the same exemplary choice ofvariables chosen for FIG. 30. As can be seen, the ramp starts after theblanking period, but the onset of LED illumination is delayed by a timet_(D). The display of the present invention provides for variation ofthe delay duration according to temperature sensed by temperature sensor280, through the action of control block 116. For example, if the LEDdelay were kept constant, as the display temperature was reduced, thefalling edge of the pixel FLC optical response would occur at later andlater time, even when the pixel image value was zero, as shown in thefigure. This would cause an undesirable amount of light to be reflectedby the pixel, degrading achievable contrast ratio. This delay in FLCresponse can be compensated, according to the present invention, byincreasing the delay in LED illumination relative to the start of theramp. Values for desired LED delay times at various expectedtemperatures can be stored in EEPROM 126, further allowing the displayof the present invention to programmably compensate for thecharacteristics of different FLC materials that might be used with it.

Pixel Drive Voltage

Another way to compensate for varying display characteristics isdescribed with reference to FIG. 32, which shows examples of thewaveform of voltage ΔV across the pixel during a portion of a phase fortypical low-temperature and high-temperature operation. At lowtemperature the FLC switches relatively slowly, and higher drivevoltages are desired to increase its speed. Conversely, at hightemperature it switches relatively fast, and lower drive voltages areoptimum. As described previously with respect to the boost cell, thevoltage V_(PIX) can be varied through a range from lower than to higherthan V_(DD). As shown in the figure, V_(PIX)=4.2 V might be chosen forlow temperature operation. Similarly, the voltage V_(WIN) applied to thecommon electrode on the glass window can be varied, and stepped throughdifferent values within a phase. FIG. 32 shows the voltage V_(WIN) setto 0.9 V during blanking, and the set to 2.6 V. The pixel electrode isdriven to 0 V or to V_(WIN) according to the state of the boost cellregister. The resulting voltage ΔV across the liquid crystal is shown tohave a high value of +3.3 V during blanking to ensure that the desiredinitial state is obtained quickly. Then, during the part of the phaseduring which the gray scale modulation is occurring, the pixel ΔV isdropped to +1.6 V to hold the already-obtained ON state. When thecomparator detects equality for the illustrated pixel and changes thestate of its boost register, the pixel electrode voltage falls to 0 V,resulting in a relatively high ΔV =−2.6 V applied across the modulator.In contrast, at high temperature, V_(PIX) might be set to a lower 2.0 V,and V_(WIN) might be set 1.0 V, and held constant. In this case the ΔVacross the pixel starts out at +1.0 V during blanking, and remains atthat level until its boost-cell register is changed, at which point itchanges to −1.0 V.

Low-power Operation

There are at least two other modes that provide for further powersavings. First of all, it is possible that power could be kept appliedto the microdisplay yet it would not receive any new image data sent toit nor would it continue to display any images. At some subsequent time,a command could be given to resume displaying images and the imagestored in the SRAM memory cells that constitute the frame buffer couldbe displayed even without new image data being sent to the microdisplay.It can be appreciated that this would possibly result in power savingsin the device that the microdisplay was located in or power savings in adevice that had previously been transmitting image data to that devicein the case of the microdisplay being located at the receiving end of acommunication link. It would also result in some power savings by themicrodisplay itself as there would be no clock or data reads or writeswhile no image was being displayed. A second power-saving mode mightinclude the display continuing to display data without receiving newimage data. This could occur in a digital still camera application, forexample, where there might be no need for the camera to continue to sendthe same image data to the microdisplay while a previously-capturedimage was being displayed for review by the operator. The power savingsin this mode would be primarily in the camera, but the microdisplaywould also have some power savings here since no new data would need tohave gamma correction, scaling, encoding, and writing to the framebuffer.

Summary of Advantages

The microdisplay circuitry and gray-scale scheme described above havesubstantial advantages over the prior art. Pulse-width modulation drive,with a limited number of pixel-drive transitions per image field, isimplemented fully digitally. “DEF” image compression reduces the numberof bits needed to be stored per pixel, without requiring thecomplexities of circuitry required for more sophisticated compressionalgorithms such as JPEG, or even canonical 4:1:1 YUV. Furthermore, thedigital comparator needed to implement PWM is shared between many pixelsrather than being implemented in each pixel. In these ways, thecomplexity of the display is minimized. According to the presentinvention, as the number of pixels in the display is increased, thenumber of transistors needed to be added per pixel (but not to thepixel) is equal to 144, i.e. 24 six-transistor SRAM registers per pixelcomprising a double-buffered image storage of twelve bits per frame plus14 transistors for the pixel boost cell, for a total of 158. Alternativevariants of the DEF scheme reduce the number of bits required per pixelfrom 12 to as low as 10, further reducing the per-pixel complexity to120+14=134 transistors. As described above in the background of theinvention, straightforward implementation of a digital PWM architecturewithout image compression results in a display complexity of 772transistors per pixel if 24-bit color is desired. Thus, microdisplaysaccording to the present invention have substantially reduced complexitycompared to other all-digital PWM implementations. Specifically, thepresent invention includes an improved design that requires only amarginal additional transistor count of less than 700, less than 600,less than 500, less than 400, less than 300, less than 200, less than160, less than 150, less than 140, and less than 135. The simpler pixelsof the present invention translate directly into smaller achievablepixels sizes, and accordingly reduced die sizes, higher silicon yields,and reduced backplane fabrication costs. Compared to the straightforward24-bit digital PWM implementation, the microdisplay of the presentinvention has approximately five times fewer transistors associated witheach pixel, resulting in 2.25-times finer achievable pixel pitch.

These advantages may be illustrated by comparative examples. Theapplicant implemented a microdisplay according the present invention asa 432×240 array of pixels in a 0.25 μm CMOS process. In this exemplaryimplementation, the pixels had a width of 12.0 μm, and a height of 16.2μm, giving active area 82 a width of 5.184 mm and a height of 3.888 mm.In this exemplary implementation the height of active area 82 plus theadditional SRAM areas 88 and 90 was 5.896 mm, which included a few sparerows of SRAM registers to provide redundancy. Each SRAM register in thisimplementation occupied a 2.74 μm×3.60 μm cell. Thus, the areaassociated with a pixel was (5184 μm×5896 μm)/(432×240)=295 μm²/pixel.This can be compared to the area per pixel in a microdisplay accordingto the prior art described above, which pixel requires 772 transistors.If this pixel were implemented in the same 0.25 μm CMOS process, andwere implemented as densely as typical SRAM, which according toestimates provided above would require a cell of area 130(0.25 μm)² forevery six transistors, then each pixel would require an area of 1045μm², not counting additional transistors that would be needed to providepixel boost cells in this low voltage (2.5 V) CMOS process.

In this exemplary implementation, the CDP of the present microdisplayrequired 8846 transistors per slice, which amounts to about 2950transistors per column, once accounting is made for the fact that aslice is either the upper half or lower half of a six-pixel-column-widepiece of the display. Thus, adding the CDP adds approximately2950/240≈12 transistors per pixel. In applicant's exemplaryimplementation each CDP had a height of about 350 μm. If the 700 μmheight of both CDPs is added to the 5.896 mm array height, to yield atotal height of 6.6 mm, the total array area per pixel is increased to330 μm², still a very considerable area savings over the 1045 μm² areaneeded for the prior art type of double-buffered digital gray-scaledisplay.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and skill and knowledge of the relevant art, are withinthe scope of the present invention. The embodiments describedhereinabove are further intended to explain best modes known ofpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other embodiments and with variousmodifications required by the particular application(s) or use(s) of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

What is claimed is:
 1. A display device, comprising: an illuminationsource; an array of active pixels, the array of active pixelselectronically controllable to switch between a first light modulatingstate that substantially passes incoming light to a user and a secondlight modulating state that substantially blocks the incoming light fromreaching the user; and a control unit coupled to operate the array ofactive pixels using digital pulse width modulation to create gray scaleimage output of the array of active pixels by comparing pixel image datavalues with a reference signal, the pixel image data values based oninput image data values, and wherein the control unit varies anintensity of the illumination source based on the reference signal toproduce a gamma characteristic in the image output.
 2. The displaydevice of claim 1, wherein the illumination source comprises alight-emitting diode.
 3. The display device of claim 1, wherein theillumination source comprises red, green, and blue light-emittingdiodes.
 4. The display device of claim 1, wherein the control unit rampsthe intensity of the illumination source in concert with the referencesignal.
 5. The display device of claim 1, wherein the control unit rampsthe intensity of the illumination source using pulse-width-modulation.6. The display device of claim 1, wherein the control unit varies theintensity of the illumination source to produce a gamma characteristicin the image output of approximately
 2. 7. The display device of claim1, wherein the array of active pixels includes a layer of liquid crystalmaterial that rotates polarization of the incoming light.
 8. The displaydevice of claim 7, wherein the layer of liquid crystal materialcomprises ferroelectric liquid crystal material.
 9. The display deviceof claim 1, the array of active pixels further comprising reflectivepixel elements, and wherein the array of active pixels reflects theincoming light in the first modulating state such that it issubstantially passed to the user and reflects the incoming light in thesecond modulating state such that it is substantially blocked fromreaching the user.
 10. The display device of claim 1, the array ofactive pixels further comprising liquid crystal material, wherein theliquid crystal material of the array of active pixels passes incomingpolarized light in the first modulating state with the same linearpolarization, and wherein in the second light modulating state theliquid crystal material rotates the linear polarization of the incominglight by 90 degrees.
 11. The display device of claim 1, wherein a colorimage is displayed during a display field, the display field comprisingdisplay phases corresponding to component colors of the color image. 12.The display device of claim 11, wherein each display phase comprises aplurality of time slots corresponding to gray scale values, and whereinduring each time slot image data is read from an array of memory cellsand compared with the reference signal.
 13. A display device,comprising: an illumination source; an array of active pixels, the arrayof active pixels electronically controllable to switch between a firstlight modulating state that substantially passes incoming light to auser and a second light modulating state that substantially blocks theincoming light from reaching the user; and control circuitry coupled tocontrol the array of active pixels to generate gray scale image outputof the array of active pixels using pulse width modulation waveformsduring a display phase, wherein the control circuitry operates the arrayof active pixels in the first light modulating state at the start of thedisplay phase and switches operation the array of active pixels to thesecond light modulating state during the display phase after adetermined time period based on input image data values, and wherein thecontrol circuitry varies an intensity of the illumination source duringthe display phase to produce a gamma characteristic in the image output.14. A method of producing a gamma characteristic in a display system,the display system comprising an illumination source and a displaydevice having an array of active pixels operable to switch between aplurality of light modulating states, the method comprising: modulatingthe array of active pixels between a first light modulating state and asecond light modulating state to produce gray scale image output of thearray of active pixels during a display phase, wherein each of the arrayof active pixels is switched to the first light modulating state at thebeginning of the display phase and pixels of the array of pixels switchto the second light modulating state after a determined time period thatis proportional to gray scale values of input image data correspondingto the pixels; and varying an intensity of the illumination sourceduring the display phase to produce a gamma characteristic in the image.15. The method of claim 14, wherein the intensity of the illuminationsource is at a minimum at the beginning of the display phase andincreases during the display phase.
 16. The method of claim 15, whereinthe intensity of the illumination source increases linearly during thedisplay phase.